Method for encapsulating electrical equipment



July 7, 1959 w. M. TERRY, JR 2,893,061

METHOD FOR ENCAPSULA'TING ELECTRICAL EQUIPMENT Filed April 5, 1957 91%.5mg 3/1. 1 M 2 m METHOD FOR ENCAPSULATING ELECTRICAL EQUIPMENT WilliamM. Terry, In, Pittsburgh, Pa., assignor to Allis- Chalmers ManufacturingCompany, Milwaukee, Wis.

' Application ApriI'S, 19 57,Serial 651,074 J 2 Claims. 01. 18-59) Thisinvention relates to a method for encasing an electrical conductingmember and more particularly to a new and improved method forencapsulating electrical apparatus.

One method in the art of encapsulating an electrical apparatus involvesintroducing a nonmetallic substance into a mold containing theelectrical apparatus in such a manner that the final product is free ofair gaps and thereby has high dielectric strength.

One way to encapsulate a transformer is to mold a casing of a solidsetting electrical insulating compound, preferably a thermosetting orthermoplastic compound around it in normal atmosphere. This methodresults in entrapment of air bubbles in the compound of the moldedtransformer between current carrying parts. When there is excessivevoltage stress on these bubbles ionization or flashover of the air willoccur. This causes intense heat which may cause a breakdown of theinsulation. This ionization also causes radio interference. To decreasethis tendency of dielectric ionization or corona, the transformer isvibrated after it has been molded. This vibration of the transformerprior to the final curing of its molded insulating casing causes most ofthe air in the compound to be eliminated. Consequently, the insulationof the transformer has a high dielectric strength. However, all of theair is not always removed by this method and the high expense of thevibrating machines is also a disadvantage for this method. Transformershave also been encapsulated in a vacuum. However, this is an expensiveand complex procedure, for filter equipment must be used to prevent thevacuum from lifting the catalyst out of the thermosetting compoundthereby upsetting its formulation.

The present invention provides an economical and relatively simplemethod for making an improved transformer with insulation having a highdielectric strength. The transformer is placed in a closed tankcontaining a gas having a dielectric strength greater than that of air.Consequently, any bubbles entrapped in the insulating compound will havea high dielectric strength and will have a high resistance to flashover.

It is, therefore, one object of the present invention to provide a newand improved method for encapsulating transformers.

Another object of the invention is to provide a new and improved methodfor making a transformer which will have a decrease in corona and willhave a high dielectric strength.

Another object of the invention is to provide a method for encapsulatinga transformer which will have increased insulation life.

Another object of the invention is to provide a new and improved methodfor encapsulating transformers which will have a decrease in ionizationand, therefore, will cause less interference with radios.

Objects and advantages other than those set forth above will be apparentfrom the following description when read in connection with theaccompanying drawing in which:

The figure is a diagrammatic vertical section of the" encapsulatingapparatus.

Referring more particularly to the drawing by 'characters of reference,the figure illustrates a preferred apparatus for encapsulating atransformer-in which a vacuum chamber 11 is provided which is ofsuflicient size to adequately contain a mold 12 having a cavity 13 and atransformer 14. The chamber 11 is provided with a pump 16 for evacuatingthe' chamber, an inlet 17 through which the solid setting insulatingcompound may be poured into the mold 12 and an inlet 18 and outlet 19for the gas. The chamber 11 must be sufiiciently sealed in order that itmay be evacuated.

After the transformer 14 has been assembled it is placed in the cavity13 of the mold 12 and then the mold and transformer are put into thechamber. After the transformer 14 and mold 12 are placed into thechamber 11, the chamber is tightly sealed and substantially evacuated ofits air by means of the pump 16. Upon attaining a near vacuum in thechamber, a gas having a dielectric strength greater than that of air isreleased from a supply tank 21 into the chamber and is exposed to thetransformer. The pressure of the gas in the chamber is brought up toatmospheric pressure or slightly above.

The dielectric strength of air as determined by the voltage stressrequired to produce sparkover of a .001 inch sphere gap at standardatmospheric pressure is 330 volts per mil thickness. Thus, whenever thetransformer is encapsulated in a gas having a dielectric strengthgreater than 330 a better transformer will be produced as compared toone encapsulated in air. However, to obtain any substantial andappreciable difference in results the gas should have a dielectricstrength of at least 25% greater than that of air. A preferred gas issulfur hexafluoride which under similar conditions as air has adielectric strength 'value of 640 volts per mil, an increase of almostover air. This is a considerable improvement in view of the fact thatthe dielectric strengths of most thermoplastic and thermosettingcompounds used to encapsulate transformers are approximately 1000 voltsper mil for a sample of 25 mils thickness. Therefore, through the use ofsulfur hexafluoride gas instead of air the difference between thedielectric strengths of the encapsulating material and its captured gasbubbles is reduced by more than 50% After the chamber 11 has been filledwith the dielectric gas the transformer 14 is ready to be encased orencapsulated. The insulating compound is released from a container 22and poured through inlet 17 into the mold cavity 13 containing thetransformer 14. Inasmuch as the chamber 11 has been evacuated ofsubstantially all air and now contains only the sulfur hexafluoride gas,any bubbles which form in the compound during the molding process willbe composed of sulfur hexafluoride gas. Since the dielectric strength ofthis gas is closer than the dielectric strength of air to that of thecompound the ability of the bubble to resist flashover is greatlyincreased, resulting in a transformer with improved insulation strength.

The gas is removed from the chamber by a pump 23 back to the supply tank21 and the transformer 14 may now go through normal curing processeswell known in the art. The resulting product is an encapsulatedtransformer which has increased dielectric strength. With this increasedstrength the transformer will be free of corona and have a longerinsulation life since the tendency of ionization is decreased. With thisdecrease in ionization, the transformer will also cause less inter-Patented July 7, 1959 ference with radios and other wave lengthoperation equipment.

Although only one embodiment of the present invention hasbeenillustrated and described, it will be apparent;

to those skilled in the art that various changes and modifications maybe made, without departing from the spirit of the invention or from thescope of the appended claims.

What is claimed is:

1-. In a method of producing an insulated electrical conducting memberthe steps of placing said member in a mold placing said mold in a vacuumchamber, evacuating said chamber and saidmold, introducing sulfurhexafluoride gas into the chamber and said mold, and

introducing a solid setting electrical insulating compound-V into saidmold.

2. In a method of producing an insulated electrical.

conducting member the steps of placing the member in a mold, placingsaid mold in a vacuum chamber, drawing a substantial vacuum in saidchamber and said mold, introducing sulfur hexafluoride gas atsubstantially atmospheric pressure into said chamber and said mold, andencasing the member with a solid setting electrical insulating compound.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A METHOD OF PRODUCING AN INSULATED ELECTRICAL CONDUCTING MEMBERTHE STEPS OF PLACING SAID MEMBER IN A MOLD PLACING SAID MOLD IN A VACUUMCHAMBER, EVACUATING SAID CHAMBER AND SAID MOLD, INTRODUCING SULFURHEXAFLUORIDE GAS INTO THE CHAMBER AND SAID MOLD, AND INTRODUCING A SOLIDSETTING ELECTRI CAL INSULATING COMPOUND INTO SAID MOLD.